1. Technical Field
The disclosure relates generally to electrical circuits and designs, and more particularly, to structures for providing common mode feedback to a differential amplifier and related design structures.
2. Background Art
Common mode feedback of a fully differential amplifier is typically provided by a secondary amplifier (referred to as a “common mode feedback amplifier”) that compares the differential output voltages to a common mode reference voltage and feedbacks the difference (from the comparison) to the fully differential amplifier to cancel out common mode offsets. In a closed loop configuration, the input voltages to the fully differential amplifier have a restricted swing of less than plus or minus 10 mV. The pair of differential inputs can operate successfully with small differential separation of the input voltages and remain in a normal operation state. The input voltages to the common mode feedback amplifier, on the other hand, are exposed to the full differential swing of the differential output voltages of the fully differential amplifier. The swing of the differential output voltages could be plus or minus 500 mV or more. Such large swings may cause the common mode feedback amplifier to enter a non-linear operation state with a common mode gain of 0 decibel (dB). In fact, a common mode gain can be 0 dB with a differential swing of as little as approximately 100 mV.
To combat this problem, switched capacitor common mode feedback is often employed. For example, U.S. Pat. No. 5,838,200 to Opris discloses a circuit structure as shown in FIG. 1. In FIG. 1, differential amplifier 200 output voltages are shorted together during sampling phase through switches 202 and 204. During the holding phase, common mode feedback amplifier 206 is disconnected at the output thereof through switch 208. CMFB amplifier 206 output bias voltage 214 is thus commonly modulated by two differential capacitors 210, 212 connected to differential amplifier 200 output voltages.